1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication processes and, more particularly, to a quantum dot three-dimensional optical path structure and corresponding fabrication method.
2. Description of the Related Art
There are many applications for photodetection in the near infrared region (the wavelength between 0.7 micron to 2 microns), such as in fiber-optical communication, security, and thermal imaging. Although III-V compound semiconductors provide superior optical performance over their silicon (Si)-based counterparts, the use of Si is desirable, as the compatibility of Si-based materials with conventional Si-IC technology promises the possibility of cheap, small, and highly integrated optical systems.
Silicon photodiodes are widely used as photodetectors in the visible light wavelengths due to their low dark current and the above-mentioned compatibility with Si IC technologies. Further, silicon-germanium (Si1-xGex) permits the photodetection of light in the 0.8 to 1.6 micron wavelength region.
However, the SiGe alloy has larger lattice constant than the Si lattice, so film thickness is a critical variable in the epitaxial growth of SiGe on Si substrates. While a thick SiGe is desirable for light absorption, too thick of a SiGe film causes a defect generation that is responsible for dark currents. This critical SiGe thickness is dependent upon the Ge concentration and device process temperature. Higher Ge concentrations and higher device process temperatures result in the formation of thinner SiGe film thicknesses. In common practice, the SiGe critical thickness is in the range of a few hundred angstroms, to maximum of a few thousand angstroms. Once the SiGe thickness is grown beyond its critical thickness, lattice defects in SiGe are inevitable. As mentioned above, an IR photo detector built from a SiGe film with lattice defects generates large dark currents and noise.
Quantum efficiency is a measure of the number of electron-hole pairs generated per incident photon, and it is a parameter for photodetector sensitivity. Quantum efficiency is defined as:η=(Ip/q)/(Popt/hv)
where Ip is the current generated by the absorption of incident optical power Popt at the light frequency v.
FIG. 1 is a graph showing the relationship between quantum efficiency and the percentage of Ge in a SiGe film. One of the key factors in determining quantum efficiency is the absorption coefficient, α. Silicon has a cutoff wavelength of about 1.1 microns and is transparent in the wavelength region between 1.3 to 1.6 microns. The SiGe absorption edge shifts to the red with an increasing Ge mole fraction and is shown in FIG. 1. The absorption coefficient of any SiGe alloy is relatively small and the limited thickness dictated by the critical thickness further limits the ability of SiGe films to absorb photons.
As noted above, the major goals of SiGe-based photodetection are high quantum efficiency and the integration of these SiGe photodetectors with the existing Si electronics. One way to increase the optical path, and improve the quantum efficiency, is to form the optical path in the same plane as the SiGe film, along the substrate surface in which the SiGe is deposited. Thus, light propagates parallel to the heterojunction (SiGe/Si) interface. However, this optical path design necessarily limits the design of IR detectors.
The IR absorption length of SiGe is long and thus a thick SiGe layer, greater than 1 micron for example, is required to achieve high IR absorption and high quantum efficiency. However, it is very difficult to grow a defect-free thick SiGe film on a Si substrate because of the lattice mismatch between these two materials. As described in pending application SURFACE-NORMAL OPTICAL PATH STRUCTURE FOR INFRARED PHOTODETECTION, which is incorporated herein by reference, a long SiGe optical path can be formed without necessarily forming a thick SiGe film. By growing the SiGe film on the sidewall of a Si trench or pillar, any IR light entering the device and traveling along the sidewall, encounters a long optical path. A long optical path improves the quantum efficiency.
However, growing the SiGe by a blanket deposition technique results in SiGe growth on the bottom of the trenches and top of the wafer, as well as on the sidewalls. Although SiGe has a larger lattice constant than Si, it can be grown lattice-matched to Si, up to the so-called critical thickness. Consequently, SiGe grown on the sidewalls has the Si lattice constant parallel to the sidewalls, but a larger lattice constant perpendicular to them. At the same time, the SiGe grown at the bottom of the trenches and top of the wafer has the Si lattice constant parallel to those surfaces, but a larger one perpendicular to them. Crystals originating from these different surfaces consequently have defected regions where they meet.
Large two-dimensional focal plane arrays in the mid- and far-infrared region can also be made using compound semiconductors such as indium antimonide or mercury cadmium telluride detector arrays bonded to a Si chip for multiplexing. However, this approach is expensive and difficult. Another method is to use quantum well infrared photodetectors (QWIP). But QWIPs are insensitive to normal incident light.
To overcome the above-mentioned limitations associated with two-dimensional structures, and to extend performance to near-infrared range of the spectrum, stress-induced quantum dots have been considered for use in a quantum dot infrared detector (QDIP). The size and distribution of the quantum dots are controlled in accordance with the Stranski-Krastanow heteroepitaxy growth mode, using a lattice mismatch between the film and the substrate material. Two-dimensional arrays of quantum dots may be formed using lithographic techniques to form a thin dielectric pattern. Beam irradiations and thermal etching techniques are also known. A multilayer quantum dot structure can also be formed by first depositing the multiple layers, followed by a heat treatment step to induce an agglomeration process to form the quantum dots. Other techniques form colloidal nanocrystals through sintering on porous template. However, these 2D quantum dot processes are relatively complex.
To avoid the above-mentioned problems inherent with a two-dimensional interface between films, it would be advantageous if a long length SiGe optical path structure could be formed using a three-dimensional array of SiGe quantum dots.